Generation of optical signals with return-to-zero format

ABSTRACT

An optical return-to-zero (RZ) signal generator and related methods are described in which a phase modulator causes a phase change in an optical signal responsive to a transition in a driving signal, and in which an interferometer receives the optical signal from the phase modulator and generates an optical pulse responsive to that phase change. Preferably, the interferometer introduces a fixed, unmodulated time delay between its two signal paths, the fixed time delay being selected such that destructive interference occurs at an output of the interferometer when the phase of the optical signal received from the phase modulator remains constant. However, when a rising or falling edge of the driving signal causes phases changes in the optical signal, the destructive interference at the output of the interferometer is disturbed, and an optical pulse is generated. The driving signal is a differentially encoded version of an input information signal. Alternatively, the driving signal is proportional to the input information signal and the transmitted RZ-formatted optical signal is a differentially encoded version of that signal. Features for regulating the fixed time delay, features for frequency shift compensation, features for loss compensation/equalization, and integrated single-chip and multiple-chip embodiments are also described.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional App. No.60/297,882, filed Jun. 13, 2001, which is incorporated by referenceherein.

FIELD

[0002] This patent specification relates to optical fiber communicationssystems. More particularly, it relates to a method and system forgenerating a high bit-rate optical signal having a return-to-zero (RZ)format.

BACKGROUND

[0003] As the world's need for communication capacity continues toincrease, the use of optical signals to transfer large amounts ofinformation has become increasingly favored over other schemes such asthose using twisted copper wires, coaxial cables, or microwave links.Optical communication systems use optical signals to carry informationat high speeds over an optical path such as an optical fiber. Opticalfiber communication systems are generally immune to electromagneticinterference effects, unlike the other schemes listed above.Furthermore, the silica glass fibers used in fiber optic communicationsystems are lightweight, comparatively low cost, and are able to carrytens, hundreds, and even thousands of gigabits per second acrosssubstantial distances.

[0004]FIG. 1 illustrates an example of a non-return-to-zero (NRZ)formatted optical signal, a return-to-zero (RZ) formatted signal, and amodulated soliton pulse train. As described in Hecht, UnderstandingFiber Optics, 3^(rd) ed., Prentice-Hall (1999), which is incorporated byreference herein, at pp. 375-380, NRZ coding is probably the most commonformat in fiber optic communications systems. As described in Hecht,supra, solitons are optical pulses that naturally retain their shape asthey travel along an optical fiber. This is basically due to a delicatebalancing act between two competing effects that degrade thetransmission of other pulses, in particular, (i) chromatic dispersion,which stretches out pulses carrying a range of wavelengths as theytravel along a fiber, and (ii) self-phase modulation, which spreads outthe range of wavelengths as pulses pass through an optical fiber.

[0005] Soliton pulses have proved surprisingly robust in optical fibers.In a long-haul wavelength division multiplexed (WDM) opticalcommunications system, this robustness allows for increased signal powerand reduced spacing among optical amplifiers and/or regenerativerepeaters. The input pulses themselves do not necessarily have to matchthe ideal soliton shape exactly, because fiber transmission gives themthe proper soliton shape. Thus, the transmission of RZ pulses, alsoshown in FIG. 1, can often results in soliton propagation along theoptical fiber. Even if not resulting in an ideal match to solitonpropagation, the RZ pulses nevertheless generally experience improvedrobustness as compared to NRZ formatted optical signals.

[0006] Optical RZ transmitters, also termed optical RZ signalgenerators, have been developed for the purpose of receiving anelectrical information signal at R bits/sec (period=T=1/R sec) andgenerating a corresponding optical signal modulated with an RZ-formattedenvelope. The input electrical signal is most commonly provided in NRZformat. For a typical RZ transmitter, the output optical signal has acarrier frequency f_(c) and free-space carrier wavelength λ_(c)=c/f_(c)in an infrared region appropriate for optical communications, e.g.,f_(c)≈196.08 THz/λ_(c)=1530 nm. Modulation rates R for commerciallyavailable RZ transmitters are generally limited to R=10 Gbps (T=100 ps)or slower, although some systems with modulation rates up to 40 Gbps(T=25 ps) have been proposed.

[0007]FIG. 2 illustrates an RZ signal generator 202 in accordance with aprior art configuration that uses two amplitude modulators (AMs). RZsignal generator 202 comprises a first AM 204, a second AM 206, and acontinuous wave (CW) laser 203 coupled as shown in FIG. 2. First AM 204receives an optical carrier signal at frequency f_(c) from CW laser 203.First AM 204 comprises a Mach-Zehnder interferometer (MZI) having afirst path 208 and a second path 209, the first path 208 having no phasemodulator and the second path 209 having phase modulator 210 thatintroduces a phase shift θ. At an output 211, the first AM 204 isdesigned to provide the difference between the signals present on thefirst path 208 and the second path 209. The phase shift θ is modulatedby a sinusoidal electrical signal V₁ provided by a sinusoidal signalgenerator 218 having a frequency equal to the desired modulation rateR=1/T, according to the relationship θ=πV₁/V_(π), where V_(π)is a fixedvalue. As known in the art, the fixed value V_(π)is determined by thenature and amount of variable-refractive-index material used in thephase modulator 210. Generally speaking, when V₁ equals 0 there is a“zero” phase shift (compared to an arbitrary reference value), and whenV₁ equals V_(π), there is a π phase shift (compared to that referencevalue).

[0008] Included in FIG. 2 is a plot 205 of the output optical power P₁versus input electrical voltage V₁ for the first AM 204 when the inputsignal is an optical carrier signal at f_(c). When the input signal V₁is at zero, the optical signals on the first path 208 and second path209 are identical and therefore the output power P₁ is zero. When theinput signal V₁ approaches V_(π), these signals have a π phasedifference and therefore the output power P₁ is a maximum. Shown in FIG.2 are time plots of the signal V₁ and output power P₁, indicating astring of narrowed optical pulses of period T being provided to thesecond AM 206. The second AM 206 is similar to the first AM 204,comprising a first signal path 212 and a phase modulator 214 along asecond path introducing a phase shift θ that is a similar function ofvoltage applied as the phase modulator 210 of first AM 204. The secondAM 206 receives an electrical NRZ data signal (e.g., 1101) having amagnitude normalized to V_(π). The second AM 206 simply serves as a gatefor the optical pulses provided by the first AM 204, allowing a pulse topass through when the NRZ data is a “1” and causing a zero output whenthe NRZ data is a “0”. FIG. 2 also includes plots of the NRZ datasignals and the resulting output signal P_(OUT).

[0009] The RZ signal generator 202 of FIG. 2 has one or moreshortcomings that can reduce its effectiveness, especially at highermodulation rates above 10 Gbps. In particular, the pulse width of theoutput signal, measured as the time difference between successive pointsof 50% power (−3 dB) relative to the maximum of the pulse, is generallybetween 0.45T and 0.5T. The pulse width can be narrowed somewhat byadjusting the specific bias point of the first AM 204, i.e., the DCvalue of V₁ in FIG. 2, or by judiciously adjusting the amplitude of thesinusoidal component of V₁. Disadvantageously, however, output powerlevels are reduced as a result of such manipulations. Furthermore, thepulse width generally cannot be made narrower than approximately 0.42Tregardless of the output power levels. Another disadvantage is that theextinction ratio of the RZ signal generator 202, defined as the ratiobetween the maximum output signal power (during a “1”) and the minimumoutput signal power (during a “0”), will suffer substantially if theamplitude of either AM driving voltage deviates from V_(π). This isbecause, during intended “off” or zero-power intervals, the phasedifferences in the arms of the AMs will deviate from π when the voltageamplitude deviates from a zero-transmission bias point, causing unwantednon-zero output power levels during these intervals. Anotherdisadvantage is that two amplitude modulators (AMs) are required in theRZ signal generator 202 of FIG. 2. This brings about increased systemcost and complexity, each AM requiring a finely biased andelectronically controlled delay element as well as a precise signalcoupler.

[0010] The RZ signal generator 202 produces an output signal in whichthe instantaneous optical frequency f_(inst) deviates from the nominaloptical frequency f_(c) It can be shown that the frequency shiftgenerated by the RZ signal generator 202 can be expressed asf_(inst)−f_(c)=(−π/4)(R)sin(2πRt). Thus, for a 10 Gbps modulation rate,the amount of frequency shift varies sinusoidally between peaks of+/−7.85 GHz.

[0011]FIG. 3 shows an RZ signal generator 302 in accordance with a priorart configuration similar to one discussed in U.S. Pat. No. 5,625,722.RZ signal generator 302 comprises an amplitude modulator (AM) 304 havinga first path 307 and a second path 310, a phase shifting element 308being placed along the first path 307 and a phase shifting element 312being placed along the second path 310. The AM 304 receives an opticalcarrier signal at frequency f_(c) from a continuous wave (CW) laser 306.The phase shifting elements 308 and 312 are symmetric with respect toeach other around a bias phase shift, such that the phase shift element308 advances the phase of the optical signal by θ₁=πV₁/V_(π)with respectto the bias phase shift when provided with a voltage V₁, and such thatthe phase shift element 312 retards the phase of the optical signal bythat same amount when provided with the opposite voltage. Included inFIG. 3 is the resulting plot 305 of the output optical power P_(OUT)versus input electrical voltage V₁ for the AM 304 when the input opticalsignal is a carrier at f_(c).

[0012] RZ signal generator 302 further comprises a differential encoder314 for receiving the NRZ data signal and generating the input voltageV₁ therefrom, and further comprises an inverter 316 for supplying (−V₁)to the AM 304. The input voltage V₁ is normalized to the V_(π)of the AM304. As known in the art, differential encoder 314 is a binary statemachine that (i) keeps its output the same when the input is a “0”, and(ii) flips its output (0→1 or 1→0) when the input is a “1.” Included inFIG. 3 are plots of an exemplary NRZ data signal (011011), thecorresponding voltage V₁, and the corresponding NRZ envelope P_(OUT) ofthe output optical signal. As indicated in FIG. 3, the RZ signalgenerator 302 operates by causing a level shift in V₁ whenever the inputdata is a “1.” As indicated by the plot 305 of the operatingcharacteristic of AM 304, the output power is zero when the voltage V₁is at 0 or V_(π), but passes through a maximum when the voltage V₁transitions between these endpoints. Thus, when the input data is a “1”the voltage V₁ will transition between endpoints, causing an opticalpulse to be emitted. However, when the input data is a “0” there will beno transition in V₁ and no optical pulse. It should be noted that thesignal V₁ will have either the curve labeled “A” or “B” in FIG. 3depending on an initial state of the differential encoder 314, but thatinitial state will be irrelevant to the presence or absence of anoptical pulse at the output of AM 304, which will only depend on thecurrent value of the NRZ data stream. The RZ signal generator 302induces no frequency shift in the optical output signal because thedelay elements of the AM are symmetric with respect to each other.

[0013] The RZ signal generator 302 of FIG. 3 has one or moreshortcomings that can reduce its effectiveness, especially at highermodulation rates equal to and above 10 Gbps. Although the optical pulsewidth can be substantially narrower than those of FIG. 2, this pulsewidth is a direct function of the rise time and fall time of theelectrical signal V₁ being provided to the AM 304. In many practicalimplementations, the rise and fall times of the electrical signalsdriving the AM 304 can be substantially different from each other, andcan vary with time, temperature, or other operating conditions. Thiscauses instability in the output pulse energies, which depend directlyon these rise and fall times. For example, if the electrical rise timeis a first percentage greater than the electrical fall time, then thepulse energy of adjacent optical pulses will also differ by that firstpercentage, which is an undesirable result. Also, if the rise and falltimes vary by a second percentage due to changes in temperature or otheroperating condition, then the output pulse energy will also change bythat second percentage, which is an undesirable result. Theseinstabilities become increasingly problematic at high modulation ratesabove 10 Gbps, where these rise and fall time variations can becomeincreasingly prominent. Another disadvantage is that the extinctionratio of the RZ signal generator 302 will also suffer substantially ifthe amplitude of the driving voltage V₁ deviates from V_(π). This isbecause, during intended “off” or zero-power intervals, the phasedifference in the arms of the AM 304 will deviate from π when theamplitude of V₁ deviates from V_(π), causing unwanted non-zero outputpower levels during these intervals. Stated another way, with referenceto plot 305 of FIG. 3, the voltage V₁ must be maintained very close to 0or very close to V_(π)or there will be non-zero output power P_(OUT)during intended “off” intervals.

[0014] Accordingly, it would be desirable to provide an optical RZsignal generator capable of generating a reliable stream of RZ opticalpulses corresponding to an electrical information signal.

[0015] It would be further desirable to provide an optical RZ signalgenerator that can provide narrow optical pulses having increased pulsewidth stability.

[0016] It would be still further desirable to provide an optical RZsignal generator that is cost-effective in terms of the number ofhigh-cost precision components required.

[0017] It would be even further desirable to provide an optical RZsignal generator in which the pulse width can be adjustable, either atthe factory or dynamically during operation.

[0018] It would be even further desirable to provide an optical RZsignal generator in which the extinction ratio of the output opticalsignal has increased stability with respect to variations in theamplitude of the electrical signals driving its electro-opticalcomponents.

[0019] It would be still further desirable to provide an optical RZsignal generator that is readily amenable to single-chip integration,dual-chip integration, and/or integration with downstream opticalcomponents such as optical amplifiers or optical attenuators.

[0020] It would be even further desirable to provide a system and methodfor integrating a plurality of optical components includinginterferometers onto smaller substrate areas.

SUMMARY

[0021] An optical return-to-zero (RZ) signal generator and relatedmethods are provided for receiving an information signal and generatingRZ optical pulses corresponding thereto, the optical RZ signal generatorcomprising a phase modulator for causing a phase change in an opticalsignal responsive to a transition in a driving signal derived from theinformation signal, the optical RZ signal generator further comprisingan interferometer for receiving the optical signal from the phasemodulator and generating an optical pulse responsive to the phasechange. According to a preferred embodiment, the interferometer isunmodulated and introduces a fixed time delay between first and secondsignal paths thereof, the fixed time delay being selected such thatdestructive interference occurs at an output of the interferometer whenthe phase of the optical signal received from the phase modulatorremains constant. However, when a rising or falling edge of the drivingsignal causes phases changes in the optical signal, the destructiveinterference at the output of the interferometer is disturbed, and anoptical pulse is generated. The optical pulse has an amplitudecorresponding to a rate of change of the phase of the optical signalreceived from the phase modulator as approximated over the fixed timedelay of the interferometer. Accordingly, the total width of the opticalpulse generated is the sum of the fixed time delay of the interferometerand the rise or fall time of the driving signal, with the −3 dB width ofthe optical pulse being substantially less than the total width.

[0022] According to a preferred embodiment, the information signal is anelectrical signal in NRZ format, and the driving signal is adifferentially encoded version of the information signal. In this case,the transmitted optical signal has a binary pattern equal to a binarypattern of the information signal. In another preferred embodiment, thedriving signal is directly proportional to the information signal, andthe transmitted optical pulse stream has a binary pattern that is adifferentially encoded version of the binary pattern of the informationsignal. In this case, differential encoding is performed at a receivingend of a fiber span carrying the transmitted optical signal. Preferably,the fixed time delay of the interferometer is comparable to therise/fall time of the driving signal. Channels of a wavelength divisionmultiplexed (WDM) signal may each be separately modulated by individualoptical RZ signal generators according to the preferred embodiments andmultiplexed together onto a common signal. Optionally, the multiple RZsignal generators may share a common interferometer having a fixed timedelay meeting the interference criteria for each channel.

[0023] The optical RZ signal generator preferably should have a fixedtime delay in the interferometer arm that is precisely maintained,and/or a precisely maintained carrier frequency, such that a timedelay-carrier frequency product is precisely maintained. According to apreferred embodiment, a feedback control circuit is used to regulate thefixed time delay and/or carrier frequency. The feedback control systemmay regulate these parameters based on signals received from an outputsignal of the interferometer and/or based on a separate pilot carrierbeam propagated through the interferometer.

[0024] According to a preferred embodiment, a compensating phasemodulator may be provided at the output of the interferometer tocompensate for frequency shifts in the optical signal. The compensatingphase modulator is driven by a compensating signal created by invertingthe driving signal and adding a delayed version of the inverted drivingsignal to itself, the delay amount being substantially equal to thefixed time delay of the interferometer. The compensating phase modulatorthereby imposes a frequency shift that is opposite in sign andsubstantially equal in magnitude to the frequency shift of the opticalsignal as it left the interferometer, resulting in an output opticalsignal that is substantially free of frequency shift.

[0025] An optical RZ signal generator in accordance with the preferredembodiments is highly amenable to chip-level integration. In onepreferred embodiment, both phase modulators and the interferometer areintegrated onto a single lithium niobate substrate, while in anotherpreferred embodiment these elements are integrated onto a singlesemiconductor substrate such as GaAs or InP. Optionally, a variableoptical attenuator/amplifier may be included on the semiconductorsubstrate for loss compensation and/or signal equalization purposes.Optionally, the optical waveguides between successive components may befolded over to reduce the amount of substrate required. Optionally, theinterferometer may comprise a Michelson interferometer with differingarm lengths to provide a folded optical path, thereby reducing theamount of substrate required.

[0026] Advantages of an optical RZ signal generator according to thepreferred embodiments include stable optical pulse widths, narrowoptical pulse widths, and high extinction ratios. Other advantagesinclude reduced fabrication costs and modest power requirements, as thedevice is highly amenable to chip-level integration and has very few(e.g., one or two) electro-optically modulated elements.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027]FIG. 1 illustrates an example of a non-return-to-zero (NRZ)formatted optical signal, a return-to-zero (RZ) formatted signal, and amodulated soliton pulse train;

[0028]FIG. 2 illustrates a prior art optical RZ signal generator;

[0029]FIG. 3 illustrates a prior art optical RZ signal generator;

[0030]FIG. 4 illustrates an optical RZ signal generator in accordancewith a preferred embodiment;

[0031]FIG. 5 illustrates exemplary waveforms corresponding to selectedlocations of the system of FIG. 4;

[0032]FIG. 6 illustrates an optical RZ signal generator in accordancewith a preferred embodiment;

[0033]FIG. 7 illustrates exemplary waveforms corresponding to selectedlocations of the system of FIG. 6;

[0034]FIG. 8 illustrates a test configuration of an optical RZ signalgenerator in accordance with a preferred embodiment using off-the-shelfcomponents and generating a repeating-1s test pattern;

[0035]FIG. 9 illustrates an oscilloscope reading corresponding to anoutput of the test configuration of FIG. 8;

[0036]FIG. 10 illustrates an optical RZ signal generator having reducedfrequency shift in accordance with a preferred embodiment;

[0037]FIG. 11 illustrates exemplary optical envelope and frequency shiftwaveforms corresponding to selected locations of the system of FIG. 10;

[0038]FIG. 12 illustrates a multiple-channel wavelength divisionmultiplexed (WDM) optical RZ signal generator in accordance with apreferred embodiment;

[0039] FIGS. 13-15 illustrate optical RZ signal generators includingtime delay/carrier frequency control circuits in accordance withpreferred embodiments;

[0040]FIG. 16 illustrates a differential encoder portion of an opticalRZ signal generator in accordance with a preferred embodiment;

[0041] FIGS. 17-20 illustrate conceptual diagrams of optical RZ signalgenerators having components integrated onto lithium niobate and/orsemiconductor substrates in accordance with the preferred embodiments;and

[0042] FIGS. 21-22 illustrate optical integrated circuits in accordancewith the preferred embodiments.

DETAILED DESCRIPTION

[0043]FIG. 4 illustrates an optical communications link 400 comprisingan RZ signal generator 401 in accordance with a preferred embodiment.For clarity of disclosure, the optical communications link 400 and RZsignal generator 401 are described with respect to a single opticalchannel having a center wavelength λ_(c) (center frequency f_(c)). Byway of example and not by way of limitation, the optical channel atλ_(c) may have a typical WDM carrier wavelength λ_(c) of 1550 nm (f_(c)of about 193.5 THz), and may have a desired modulation rate R of 40Gbps. Optical communications link 400 further comprises a fiber span 402that may include one or more repeaters and/or regenerators (not shown)as required to transmit the optical signal to a receiver portion 403. RZsignal generator 401 comprises a continuous wave (CW) carrier source404, such as a laser, that generates an optical carrier signal accordingto Eq. (1) below:

carrier(t)=cos(2πf_(c)t)  {1}

[0044] For clarity of disclosure, it is presumed herein that the opticalcarrier signal emitted by CW laser 404 has an amplitude normalized to 1.RZ signal generator 401 further comprises a phase modulator 405, adifferential encoder 406, and an interferometer 412 coupled asillustrated in FIG. 4. Signal generator 401 receives an electrical inputsignal x(t), an NRZ signal representing the data to be encoded onto thecarrier and having an exemplary bit rate R of 40 Gbps (corresponding toa period T of 25 ps per bit). While the electrical input data signalx(t) is presumed herein to be an NRZ signal for clarity of disclosure,it is to be appreciated that the RZ signal generator 401 could bereadily adapted to receive the electrical input in RZ format or otherformats. Differential encoder 406 is a binary state machine that keepsits output the same when the input is a “0” and that toggles its outputwhen the input is a “1.” In one preferred embodiment, differentialencoder 406 comprises an exclusive-or (XOR) gate and a feedback loophaving a delay of T, i.e., the bit period of the electrical input signalx(t). However, other configurations can be used for differentialencoding as described further infra. An output x′(t) of the differentialencoder 406 is provided to the phase modulator 405.

[0045]FIG. 5 illustrates exemplary waveforms corresponding to selectedlocations of the system of FIG. 4, beginning with the NRZ signal x(t)carrying an exemplary bit stream portion “011100101.” Importantly, FIG.5 illustrates the signal x(t) in a real-world representation in having afinite rise and fall time. By way of example, the rise and fall timesare shown as being approximately 5 ps, although the preferredembodiments are applicable to a variety of real-world input signals x(t)having different rise and fall times. Also illustrated in FIG. 5 is areal-world representation of the signal x′(t) output from thedifferential encoder 406. Importantly, the signal x′(t) will change itsstate (i.e., transition from OFF→ON or ON→OFF) when the input signalx(t) is a “1”, but will not change its state when the input signal x(t)is a “0”. Like all real-world signals, x′(t) has a finite rise timet_(rise) and a finite fall time t_(fall). As described below, it isstate transitions of the differential encoder output x′(t) over timeintervals t_(rise) and t_(fall) that trigger the formation of opticalpulses from the RZ signal generator 401. Advantageously, in thepreferred embodiment of FIG. 4, the durations of the rise and fall timest_(rise) and t_(fall) are not associated with externally providedsignals, but rather are associated with the hardware of the differentialencoder 405 that produces x′(t). In this sense, the rise and fall timest_(rise) and t_(fall) are more controllable by the manufacturer of theRZ signal generator than if the signal x′(t) was provided from externalelectrical sources. As described infra, the rise and fall times t_(rise)and t_(fall) contribute to the width of the emitted optical pulsesalthough, advantageously, they are not the sole determining factors ofthe emitted pulse width. By way of example, the rise and fall timest_(rise) and t_(fall) may be each maintained at about 5 ps, although thepreferred embodiments are applicable to a variety of different rise andfall times.

[0046] Phase modulator 405 comprises a phase shift element having adifferential delay that is proportional to the electrical input signalx′(t), thereby causing a differential phase shift that is proportionalto the electrical input signal x′(t) over the wavelengths of interest.By way of example, the delay element may comprise a substrate materialsuch as lithium niobate (LiNbO₃) whose index of refraction can vary withapplied voltage. As known in the art, the phase modulator 405 has acharacteristic parameter V_(π)corresponding to the voltage for which itinduces a phase shift of π radians for a carrier beam of a specifiedfrequency f_(c). For a typical lithium niobate phase modulator, V_(π)isapproximately 5 volts. However, V_(π)can be as low as 1 volt for somephase modulators such as those based on indium phosphide (InP)substrates. Phase modulator 405 receives the carrier signal of Eq. (1)and modulates it with the differentially encoded signal x′(t) to producean output y(t) according to Eq. (2) below. $\begin{matrix}{{y(t)} = {{\cos \left( {{2\pi \quad f_{c}t} - \theta} \right)} = {{\cos \left( {{2\pi \quad f_{c}t} - {a\quad {x^{\prime}(t)}}} \right)} = {\cos \left( {{2\pi \quad f_{c}t} - {\pi \frac{x^{\prime}(t)}{V_{\pi}}}} \right)}}}} & \left\{ 2 \right\}\end{matrix}$

[0047] In Eq. (2), the value “a” represents an efficiency metric of thephase modulator 405 which, in accordance with a preferred embodiment, isselected such that ax′(t)=π when x′(t) is equal to V_(π), i.e.,a=π/V_(π). Accordingly, when x′(t) is zero, optical signal y(t) has aphase lag of zero, and when x′(t) is V_(π), optical signal y(t) has aphase lag of π.

[0048] Interferometer 412 comprises first and second couplers 418 and420 and a delay element 416 having an optical delay of τ. The amount ofoptical delay τ is judiciously selected according to a first set ofcriteria for generation of RZ formatted signals as described herein.Generally, for the generation of RZ formatted signals, the optical delayτ is a fraction of the bit period T. As known in the art, theinterferometer 412 will generate a first output z(t) according to Eq.(3) below, and a second output Z_(ALT)(t) according to Eq. (4) below:$\begin{matrix}{{z(t)} = {{{y(t)} - {y\left( {t - \tau} \right)}} = {{\frac{1}{2}{\cos \left( {{2\pi \quad f_{c}t} - {\pi \frac{x^{\prime}(t)}{V_{\pi}}}} \right)}} - {\frac{1}{2}{\cos \left( {{2\pi \quad f_{c}t} - {2\pi \quad f_{c}\tau} - {\pi \frac{x^{\prime}\left( {t - \tau} \right)}{V_{\pi}}}} \right)}}}}} & \left\{ 3 \right\} \\{{z_{A\quad L\quad T}(t)} = {{{y(t)} + {y\left( {t - \tau} \right)}} = {{\frac{1}{2}{\cos \left( {{2\pi \quad f_{c}t} - {\pi \frac{x^{\prime}(t)}{V_{\pi}}}} \right)}} + {\frac{1}{2}{\cos \left( {{2\pi \quad f_{c}t} - {2\pi \quad f_{c}\tau} - {\pi \frac{x^{\prime}\left( {t - \tau} \right)}{V_{\pi}}}} \right)}}}}} & \left\{ 4 \right\}\end{matrix}$

[0049] Of course, Eq. (4) above may be recast as Eq. (5) below:$\begin{matrix}{{z_{A\quad L\quad T}(t)} = {{{y(t)} + {y\left( {t - \tau} \right)}} = {{\frac{1}{2}{\cos \left( {{2\pi \quad f_{c}t} - {\pi \frac{x^{\prime}(t)}{V_{\pi}}}} \right)}} - {\frac{1}{2}{\cos \left( {{2\pi \quad f_{c}t} - \left\lbrack {{2\pi \quad f_{c}\tau} - \pi} \right\rbrack - {\pi \frac{x^{\prime}\left( {t - \tau} \right)}{V_{\pi}}}} \right)}}}}} & \left\{ 5 \right\}\end{matrix}$

[0050] For the preferred embodiment of FIG. 4 in which the output z(t)of interferometer 412 is used, one criteria for the delay value τ isgiven by Eqs. (6)-(7) below:

2πf_(c)τ=2πm, or  {6}

τ=(1/f_(c))m=(λ_(c)/c)m  {7}

[0051] In Eq. {7}, “c” is the speed of light and “m” is a positiveinteger. For an alternative preferred embodiment in which the outputz_(ALT)(t) of interferometer 412 is used, the criteria for the delayvalue τ is given by Eqs. (8)-(9) below:

2πf _(c)τ−π=2πm, or  {8}

τ=(1/f _(c))(m+½)=(λ_(c) /v)(m+½)  {9}

[0052] If the criterion of Eq. (7) is satisfied for the preferredembodiment of FIG. 4 (or the criterion of Eq. (9) is satisfied ifZ_(ALT)(t) is used instead of z(t)), then the equation for the signaltransmitted for either case is given by Eq. (10) below: $\begin{matrix}{{z(t)} = {{\frac{1}{2}{\cos \left( {{2\pi \quad f_{c}t} - {\pi \frac{x^{\prime}(t)}{V_{\pi}}}} \right)}} - {\frac{1}{2}{\cos \left( {{2\pi \quad f_{c}t} - {\pi \frac{x^{\prime}\left( {t - \tau} \right)}{V_{\pi}}}} \right)}}}} & \left\{ 10 \right\}\end{matrix}$

[0053] This expression for the transmitted optical signal z(t) can berecast into carrier-envelope form using trigonometric identities,resulting in Eq. (11) below: $\begin{matrix}{{z(t)} = {{\sin \left( {{2\pi \quad f_{c}t} - {\pi \frac{{x^{\prime}(t)} + {x^{\prime}\left( {t - \tau} \right)}}{2V_{\pi}}}} \right)}{\sin \left( {\pi \frac{{x^{\prime}(t)} - {x^{\prime}\left( {t - \tau} \right)}}{2V_{\pi}}} \right)}}} & \left\{ 11 \right\}\end{matrix}$

[0054] Thus, when the condition of Eq. (7) is satisfied, it is readilyseen from Eq. (11) that the signal transmitted across fiber span 402 issimply an optical carrier at f_(c) having an envelope ofsin{π[x′(t)−x′(t−τ)]/2V_(π)]. In particular, when x′(t) is driven atvoltages for which the envelope sine term is a monotonic function (e.g.,when x′(t) remains between zero and V_(π)), the carrier envelopemagnitude roughly corresponds to the derivative of x′(t) approximatedover an interval of τ. Accordingly, any transition in x′(t) (OFF→ON orON→OFF), which in turn corresponds to an input data value x(t)=1, willcause an optical pulse to be transmitted. In contrast, if there is notransition in x′(t), which in turn corresponds to an input data valuex(t)=0, there will be no optical pulse transmitted. The total durationof the emitted optical pulse is (τ+t_(rise)) when x′(t) rises from OFFto ON, and is (τ+t_(fall)) when x′(t) falls from ON to OFF. Defining atotal pulse width as the time difference between points of 5% maximumpower for a given pulse, it is readily seen that the total pulse widthcontinually alternates between approximately (τ+t_(rise)) andapproximately (T+t_(fall)).

[0055]FIG. 5 further illustrates a conceptual example of the transmittedsignal z(t), which is exaggerated in that there are actually tens ofthousands of optical signal cycles at f_(c) within a given period T ofthe signal x′(t). As indicated in FIG. 5, the signal z(t) is simply alight beam at f_(c) modulated by an approximate derivative of the signalx′(t) (scaled by τ), and corresponds to an RZ-modulated version of theoriginal input bit stream x(t). When demodulated by any suitabledemodulator 403, the resulting signal w(t) has the same bit pattern asthe original binary signal x(t).

[0056] The value of the delay τ of the interferometer 412 determinesseveral performance characteristics of the signal generator 401. First,the delay τ represents a lower boundary of the total width of theemitted optical pulses. As described supra, the total pulse width isequal to (τ+t_(rise)) or (τ+t_(fall)) as appropriate. The correspondingoptical pulse width as measured by the −3 dB or 50% point will besubstantially narrower, of course, than the total pulse width.Advantageously, because the delay τ is a fixed value, the pulse widthand optical pulse energy is appreciably stabilized with respect tovariations in the rise and fall times of the electronic circuitrydriving the electro-optical components. This is especially advantageousover the prior art configuration of FIG. 3 supra, in which the pulsewidth depends entirely on the rise and fall times t_(rise) and t_(fall),and there is no lower boundary which can be problematic at lowermodulation rates. In contrast, the total pulse width of the RZ signalgenerator 401 has a lower boundary at τ, and any changes Δt_(rise) orΔt_(fall) will only result in a change of approximatelyΔt_(rise)/(τ+t_(rise)) or Δt_(fall)/(τ+t_(rise)) in the total pulsewidth.

[0057] Second, the delay τ of the interferometer 412 also affects theoutput optical pulse magnitude. In particular, if τ is very smallcompared to the bit period T (e.g., 1%) such that τ is very smallcompared to the rise/fall time of the signal x′(t), the RZ pulsestrength will be small because the difference x′(t)−x′(t−τ) will besmall. It is generally preferable that τ be comparable to the rise andfall times t_(rise) and t_(fall) of the signal x′(t) such that thedifference x′(t)−x′(t−τ) will become appreciable over pulse widthinterval. By way of example, one suitable value for τ would be about 5ps for the above example in which t_(rise)=t_(fall)=5 ps, although othervalues may result in adequate performance as well. However, as τincreases past about 50% of the bit period T, the performance degradesbecause the difference x′(t)−x′(t−τ) begins to lose its meaning as beingindicative of a derivative of x′(t). Advantageously, according to apreferred embodiment, τ is a fixed delay built into the interferometer412 and so does not require modulation. Thus, only a single element (thephase modulator 405) requires electro-optical modulation, loweringfabrication costs and increasing reliability.

[0058] Notably, if τ were increased to about 100% of the bit period T,it can be readily shown that the transmitted signal z(t) would take onan envelope that is an NRZ formatted version of the original signalx(t). Thus, according to an optional preferred embodiment, the signalgenerator 401 may be configured as an NRZ optical modulator by settingthe delay τ of the interferometer 412 equal to the bit period T.

[0059] In addition to being judiciously selected, the delay τ should bemaintained with sufficient precision. In particular, letting τ=τ₀+Δτwhere T₀ is the exact value that satisfies Eq. (7), it is preferablethat the deviation Δτ is sufficiently small such that 2τf_(c)Δτ does notexceed about 0.1 radians. The carrier frequency f_(c) must also bemaintained with sufficient precision such that Eqs. (6)-(7) supra aresatisfied.

[0060] For clarity of disclosure, and not by way of limitation, anumerical design example is presented. Let f_(c) be equal to a standardWDM carrier frequency of 193.0 THz, let the bit rate of x(t) be 40 Gbps(bit period=25 ps), and let the rise/fall time of the electrical signalx′(t) again be 5 ps. To get the delay τ within a neighborhood comparableto the rise/fall time, one suitable value for “m” is 965, which yields adelay τ of 5.000000 ps. To maintain the value 2τf_(c)Δτ less than 0.1radians, the deviation Δτ should remain less than 0.000082 ps, that is,the delay τ needs to be maintained between 5.000000 ps and 5.000082 ps.In accordance with another preferred embodiment, the delay τ may beregulated by a feedback control circuit to maintain the proper delaytolerances as will be described further infra.

[0061] In addition to advantageously providing increased pulse widthstability, the RZ signal generator 401 also provides narrower pulsewidths and improved extinction ratios. By way of example, ifτ=t_(rise)=t_(fall)=5 ps for a modulation rate R=40 Gbps (T=25 ps), thetotal pulse width will be 10 ps or 40% of the bit period, and the −3 dBpulse width will be substantially narrower (e.g., 20-30% of the bitperiod). By way of further example, for a modulation rate of R=10 Gbps(T=100 ps), even more relaxed electrical parameters such ast_(rise)=t_(fall)=15 ps will yield a total pulse width of 30 ps=30%(where τ is set to 15 ps), and the −3 dB pulse width will besubstantially narrower (e.g., 10-20% of the bit period). The extinctionratio of the RZ signal generator 401 is also substantially improvedbecause the delay τ of the interferometer is fixed, rather thanmodulated. The fixed delay τ, which corresponds to a phase difference of2πm as described supra, will invariably cause a precise zero-poweroutput as long as the signal x′(t) is not changing, regardless of theactual value of x′(t) at that time. Thus, even if the amplitude of thevoltage driving the phase modulator 405 deviates from V_(π), or ifenvironmental factors cause a change in the induced signal delay of thephase modulator 405 when driven at V_(π), the “zeroes” in the outputsignal z(t) will remain pure because it is changes in the phase θ, andnot the absolute value of the phase θ, that cause the presence of energyin the output signal. For these reasons and other reasons, the operationof the RZ signal generator 401 is amenable to operation at substantiallyhigher bit rates, such as 40 Gbps, where conventional RZ signalgenerators that rely on AM devices are less effective.

[0062]FIG. 6 illustrates an optical communications link 600 comprisingan RZ signal generator 601 in accordance with a preferred embodiment,wherein differential encoding is to be performed at the receiving endrather than the source end of the fiber span. RZ signal generator 601 issimilar to the RZ signal generator 401 of FIG. 4 except that it containsno differential encoder. Rather, the input NRZ data signal (normalizedto V_(π)instead of 1) is provided directly to the phase modulator 405.Optical communications link 600 comprises a differential encoder 603 atthe output of a demodulator 403 adapted to receive a stream w_(I)(t) ofRZ optical pulses and generate a differentially-encoded NRZ signal w(t)therefrom.

[0063]FIG. 7 illustrates exemplary waveforms corresponding to selectedlocations of the system of FIG. 6, beginning with the NRZ DATA signalhaving values of “011100101” (normalized to an amplitude of V_(π)). Asindicated therein, the signal z(t) transmitted across the fiber span hasan envelope that is a differentially-encoded, RZ-formatted version ofthe NRZ DATA signal. The original NRZ DATA signal bit stream isreproduced at the output w(t) of the differential encoder 603. Unlikethe preferred embodiment of FIG. 4, the preferred embodiment of FIG. 6has a pulse width that depends in part on the rise and fall timest_(rise) and t_(fall) of an externally provided data signal. Like thepreferred embodiment of FIG. 4, however, the preferred embodiment ofFIG. 6 exhibits pulse width stability and extinction ratio stability.Also, the output pulse widths are similarly narrower when the rise andfall times t_(rise) and t_(fall) are maintained within expectedtolerances.

[0064]FIG. 8 illustrates a test configuration 800 of an optical RZsignal generator in accordance with a preferred embodiment that usedoff-the-shelf components and generated a repeating-1s test pattern(i.e., a pattern of 1, 1, 1, . . . ,1). Test configuration 800 compriseda standard CW laser 802, an off-the-shelf lithium niobate phasemodulator 804, and an off-the-shelf Mach-Zehnder interferometer 806. Inparticular, phase modulator 804 comprised part number PM-00-12-PFA-PFA,available from EOSPACE, Inc. of Redmond, Washington, and interferometer806 comprised a 25 GHz Interleaver, part number CFOI0250000, availablefrom Oplink Communications, Inc. of San Jose Calif. having a fixed delayarm of τ=20 ps. An electrical signal IN(t) of alternating ones andzeroes having a bit period T=100 ps (R=10 GHz) was provided.

[0065]FIG. 9 illustrates an oscilloscope reading 900 corresponding anoutput of the test configuration of FIG. 8. As indicated in FIG. 9, arepeating-1s test pattern was generated, with each optical pulse havinga −3 dB width of less than about 35 ps, i.e., less than about 35% of thebit period T.

[0066] The RZ signal generators 401 and 601 of FIGS. 4 and 6,respectively, produce a output signals in which the instantaneousoptical frequency f_(inst) deviates from the nominal optical frequencyf_(c) The frequency shift alternates in sign between successive opticalpulses, i.e., one pulse has a positive frequency shift, the next pulsehas a negative frequency shift, the next pulse has a positive frequencyshift, and so on (see FIG. 11, infra). This frequency shifting behavioris distinguished from “chirp,” a term that is generally associated withoptical pulses in which a positive frequency shift takes place on therising side of the pulse and a negative frequency shift takes place onthe falling side of the same pulse. Such frequency shift might bedesirable in some optical communications applications, e.g., in certainlong-haul wavelength division multiplexed (WDM) links in which it isdesired to use a single bulk dispersion compensation element at the endof the link. However, the presence of such frequency shift is notdesirable in other optical communications applications, e.g., when it isdesired to distribute several dispersion compensating elements atsuccessive points along the link. Accordingly, it may be desirable tocontrol and/or minimize the amount of such frequency shift in the outputsignal.

[0067]FIG. 10 illustrates an optical RZ signal generator 1000 havingreduced frequency shift in accordance with a preferred embodiment. Asindicated by Eq. 11 supra, the output z(t) of the RZ signal generator401 of FIG. 4 contains a frequency shift term in its carrier component,and it may be desirable to reduce or eliminate the amount of thisfrequency shift in practical applications. RZ signal generator 1000comprises a CW laser 1004 similar to the CW laser 404 of FIG. 4, a phasemodulator 1005 similar to the phase modulator 405 of FIG. 4, aninterferometer 1012 similar to the interferometer 412 of FIG. 4, and adifferential encoder 1006 similar to the differential encoder 406 ofFIG. 4. For a given NRZ data signal x(t), the output of interferometer1012 is identical to the output z(t) of FIG. 4. RZ signal generator 1000further comprises a frequency shift compensator 1042 designed to receivethe optical signal z(t) and the driving signal x′(t), and to generate anoutput signal z₁(t) having reduced or eliminate frequency shift.

[0068]FIG. 11 illustrates exemplary optical envelope and frequency shiftwaveforms corresponding to selected locations of the system of FIG. 10.When the optical signal z(t) has a “1” RZ pulse, it has a frequencyshift FSHIFT_(z)(t) in a first direction (the positive direction, forexample) with a magnitude that sharply increases toward the modulationrate R and then decreases back to zero, as indicated in FIG. 11. For thenext pulse, the frequency shift is in the opposite direction (negative,for example) with a similar magnitude profile. Generally speaking, for abit rate R, the uncompensated signal z(t) will have a frequency shiftmagnitude of about +/−R, as indicated in FIG. 11. Thus, for a 10 GHzmodulation rate, the frequency shift magnitude will be about +/−10 GHz,and the frequency spectrum of the optical signal z(t) will displaysideband power at f_(c)+10 GHz and f_(c)−10 GHz.

[0069] Frequency shift compensator 1042 comprises an electrical inverter1044 coupled to receive the signal x′(t) being provided to the phasemodulator 1005, an electrical signal splitter 1045 for splitting theinverted signal along a first electrical path 1046 and a secondelectrical path, the second electrical path comprising an electricaldelay element 1048 that induces an electrical delay of τ_(c), and anelectrical signal combiner 1050 for recombining the electrical signalsas shown in FIG. 10, and a compensating phase modulator 1040.Preferably, the electrical delay τ_(c) is very close to the opticaldelay τ of the delay of the interferometer 1012. An output x₁(t) of theelectrical signal combiner 1050 is provided to the compensating phasemodulator 1040, which induces a phase delay of πx₁(t)V_(π). While theenvelope of the output signal z₁(t) remains the same as the envelope ofthe signal z(t), the carrier component will have an equation shown inEq. (12) below. In Eq. (12), a delay Δt is included, reflecting apropagation time difference between (i) an upper propagation path fromthe differential encoder 1006 through the phase modulator 1005 and theinterferometer 1012 to the compensating phase modulator 1040, and (ii) alower propagation path from the differential encoder 1006 through theelectrical signal splitter 1045 and the electrical signal combiner 1050to the compensating phase modulator 1040. $\begin{matrix}{{z_{1,{C\quad A\quad R\quad R\quad I\quad E\quad R}}(t)} = {\sin \left( {{2\pi \quad f_{c}t} - {\pi \frac{{x^{\prime}\left( {t + {\Delta \quad t}} \right)} + {x^{\prime}\left( {t + {\Delta \quad t} - \tau_{c}} \right)}}{2V_{\pi}}} + {\pi \frac{{x^{\prime}(t)} + {x^{\prime}\left( {t - \tau} \right)}}{2V_{\pi}}}} \right)}} & \left\{ 12 \right\}\end{matrix}$

[0070] Accordingly, when τ_(c) is identical to τ and Δt is equal tozero, there will be no frequency shift present in the output signalz₁(t). Because the delays 1016 and 1048 lie along two separate paths,one optical and one electrical, it is less likely that τ_(c) isprecisely identical to τ. However, as long as the difference τ−τ_(c) isvery small compared to the rise or fall time of the signal x′(t), whichis generally not a difficult objective to achieve, the amount offrequency shift in the output signal z₁(t) will be very small, asindicated in the plot FSHIFT_(z1)(t) of FIG. 11. Of course, thepropagation times along the differing optical and electrical paths tothe phase modulator 1040 should otherwise be kept identical, i.e., Δtshould be equal to zero, or their difference Δt should be kept verysmall compared to the rise or fall time of the driving signal x′(t).

[0071]FIG. 12 illustrates an N-channel WDM signal generator inaccordance with a preferred embodiment, comprising a plurality of frontends 1202 for differentially encoding and phase modulating therespective input signals x₀(t), x₁(t), . . . , x_(N−1)(t) as describedsupra with respect to like elements of FIG. 4, except that successivelydifferent carrier frequencies are used. Each differential encoders offront ends 1202 comprises an exclusive-or (XOR) gate and a feedback loophaving a delay of T, i.e., the bit period of the electrical input signalx(t). The outputs of the front ends 1202 are combined into a commonlight beam by conventional multiplexer 1204 and amplified bysemiconductor optical amplifier (SOA) 1206. Finally, the optical signalis fed to a common interferometer 1208 for providing the WDM signal z(t)having NRZ-encoded channels of data.

[0072] Importantly, the delay τ of the interferometer 1208 must bejudiciously selected such that the criteria of Eq. (7) or Eq. (9) aresatisfied for each of the signals. If a value τ can be found such thatEq. (7) is satisfied for all channels, then the embodiment of FIG. 4 inwhich a single output z(t) of interferometer 1208 is used. If thechannels are closer together and a value τ can be found only such thatalternating channels satisfy Eq. (7) and Eq. (9), respectively, thenboth inputs of the interferometer 1208 may be used, in which case allthe channels are multiplexed at one common output port prior totransmission. To summarize, if it is desired for N channels to bemodulated onto carrier frequencies of f₀+Δf_(k)(k=0, . . . , N−1), thenthere should exist integers m₀ and n_(k) (k=0, . . . , N−1) and a singlevalue τ for which either of the following equations (13) or (14) aresatisfied for a given value of k:

(ƒ₀+Δƒ_(k))τ=(m ₀ +n _(k))  {13}

[0073] $\begin{matrix}{{\left( {f_{0} + {\Delta \quad f_{k}}} \right)\tau} = \left( {m_{0} + n_{k} + \frac{1}{2}} \right)} & \left\{ 14 \right\}\end{matrix}$

[0074]FIG. 13 illustrates an RZ signal generator 1300 including a timedelay/carrier frequency control circuit in accordance with a preferredembodiment. As described supra, it is important that the value of thetime delay τ of the interferometer associated with the preferred signalgenerator, and/or the carrier frequency f_(c), be precisely maintainedfor proper operation such that Eq. (7) remains satisfied for the case inwhich the z(t) output of the interferometer is used (or Eq. (9) remainssatisfied for the case in which the Z_(ALT)(t) output of theinterferometer is used). For the description below it is assumed thatthe z(t) output is used. RZ signal generator 1300 comprises a carriersource 1304, a phase modulator 1305, and a differential encoder 1306similar to elements 404-406, respectively, of the RZ signal generator401 of FIG. 4, supra. RZ signal generator 1300 further comprises aninterferometer 1312 similar to the interferometer 412 of FIG. 4,comprising first and second couplers 1318 and 1320 and a delay element1316. RZ signal generator 1300 further comprises a time delay/carrierfrequency control circuit 1350 comprising a photodetector 1352 coupledto the alternative output Z_(ALT)(t) of interferometer 1312, and furthercomprises an electrical control circuit 1354 configured to vary thedelay τ and/or the carrier frequency f_(c) responsive to the output ofphotodetector 1352.

[0075] In the embodiment of FIG. 13, it is the subtractive output z(t)of interferometer 1312 that is output to the fiber span. Assuming f_(c)to be constant, it has been found that when the output z(t) comprisesthe desired carrier signal modulated in the RZ-format and the delay τ isat the optimal delay time τ₀, the alternative output Z_(ALT)(t) willcomprise the carrier beam modulated by an envelope that is a complementof the envelope of z(t), such that for ON bits of z(t) the signalZ_(ALT)(t) will be ON for a large portion of the duty cycle after thez(t) envelope goes low, and such that for OFF bits of z(t) the signalZ_(ALT)(t) will be ON for the whole bit period. As the delay τ wandersfrom the optimal delay time τ and the signal z(t) degrades, the signalZ_(ALT)(t) will lose power. The electrical control circuit 1354 isadapted and configured to continually adjust the delay τ so as tomaximize the average power in the signal Z_(ALT)(t). Thus, the delaycontrol circuit 1350 is a feedback control circuit that keeps the delayτ at its optimal value τ₀ to ensure proper operation of the RZ signalgenerator 1300. Since the delay τ will vary slowly (e.g., over severalseconds) as compared to the bit rate and carrier frequency, high-speedelectronics are not necessarily required in the electrical controlcircuit 1354, and any of a variety of known feedback control laws may beused to regulate the delay τ. Similar principles apply to the additionalor alternative case in which the carrier frequency f_(c) varies and iscontrolled by the time delay/carrier frequency control circuit 1350.

[0076]FIG. 14 illustrates an RZ signal generator 1400 including a timedelay/carrier frequency control circuit in accordance with anotherpreferred embodiment, wherein a pilot carrier beam tapped from thecarrier source is fed through the delay element in a direction oppositethe direction of the phase-modulated beam to provide a feedback signalto regulate the delay τ and/or the carrier frequency f_(c). RZ signalgenerator 1400 comprises a carrier source 1404, a phase modulator 1405,and a differential encoder 1406 similar to elements 404-406,respectively, of FIG. 4, supra. RZ signal generator 1400 furthercomprises an interferometer 1412 similar to the interferometer 412 ofFIG. 4, comprising first and second couplers 1418 and 1420 and a delayelement 1416 similar to delay element 416 of FIG. 4. RZ signal generator1400 further comprises a coupler 1458 adapted to tap a small amount ofsignal (e.g., 10%) from the carrier beam, the tapped beam beinghereinafter referred to as a pilot carrier beam. The pilot carrier beamis provided to the alternate output of interferometer 1412.Interferometer 1412 further comprises a node 1456 (unused in previouspreferred embodiments). The output at node 1456 is provided to aphotodetector 1452, whose output is provided to an electrical controlcircuit 1454 to control the delay τ and/or the carrier frequency f_(c)The power at node 1456 will reach a minimum when the condition of Eq.(7) is satisfied, and the electrical control circuit 1454 regulates thedelay τ and/or the carrier frequency f_(c). to maintain this minimum. Asindicated in FIG. 14, an isolator 1460 is inserted after the phasemodulator 1405 to prevent undesired introduction of the pilot carrierbeam back into the phase modulator 1405, and isolators 1461 and 1462 arealso inserted as shown in FIG. 6 to prevent undesired feedback ofsignals back into the carrier source 1404.

[0077] Advantageously, under normal operating conditions, anyperturbations in the time delay τ take place over a very long timeperiod (e.g., several seconds) compared to the information signalmodulation rate, which is well into the GHz range. Moreover, themagnitude of any changes in the time delay τ as it is being regulatedare extremely small, particularly in comparison to the time delay shiftsof modulated elements such as the phase modulator 1405. Accordingly, thetime delay τ may still be appropriately referred to as a “fixed timedelay” and treated as a true constant in Eqs. (3)-(14), supra, for theembodiment of FIG. 14.

[0078]FIG. 15 illustrates an RZ signal generator 1500 including a delaycontrol circuit 1550 in accordance with a preferred embodiment, whereinthe polarization of a pilot carrier beam tapped from the carrier sourceis rotated by 90 degrees, and then the pilot carrier beam is fed throughthe delay element of the interferometer in the same direction as thephase-modulated beam to provide a feedback signal to regulate the delayτ and/or the carrier frequency f_(c) RZ signal generator 1500 comprisesa carrier source 1504, a phase modulator 1505, and a differentialencoder 1506 similar to like elements of the signal generator of FIG.14, supra. Signal generator 1500 further comprises an interferometer1512 similar to the interferometer 1412 of FIG. 14, comprising first andsecond couplers 1518 and 1520 and a delay element 1516 similar to delayelement 1416 of FIG. 14. RZ signal generator 1500 further comprises acoupler 1558 configured to tap a small pilot carrier beam from the inputcarrier beam.

[0079] According to a preferred embodiment, polarization-maintainingfibers are used to couple the components of FIG. 15. Carrier source 1504provides a z-polarized carrier beam that maintains its polarizationthrough the phase modulator 1505, as indicated by the “C” (for carrier)followed by the z-vector symbol in FIG. 15. At first, the pilot carrierbeam also has a polarization in the z direction, as indicated by the “P”(for pilot) followed by the z-vector symbol in FIG. 15. However, thepilot carrier beam is rotated by 90 degrees by a 90-degree rotatorelement 1571 to be polarized in the y direction, as indicated by the “P”followed by the y-vector symbol in FIG. 15. The two beams are thenrecombined at a polarization beamsplitter 1570, which maintains theirrespective polarizations, and the result is provided to theinterferometer 1512. As known in the art, these two beams will notinterfere with each other in relation to the operation of interferometer1512.

[0080] The output of the interferometer 1512 is then provided to asecond polarization beamsplitter 1572, which separates it according topolarization onto two separate paths, a first path containing thez-polarized modulated carrier beam z(t), and a second path containingthe y-polarized pilot carrier beam. The y-polarized pilot carrier beamis then provided to a photodetector 1552. In a manner similar to theoperation of the device of FIG. 14, an electrical control circuit 1554will use the output of the photodetector 1552 to properly regulate thedelay τ of the delay element 1516 and/or the carrier frequency f_(c).The preferred embodiment of FIG. 15 does not require isolator elementsas in the preferred embodiment of FIG. 14, although the preferredembodiment of FIG. 14 may be advantageous in that it is not dependent onthe particular relative polarizations of the carrier beams and the pilotcarrier beams.

[0081]FIG. 16 illustrates a differential encoder 1600 that may be usedin conjunction with the preferred embodiments, comprising anoff-the-shelf NRZ-RZ gate 1602 and an off-the-shelf T flip-flop 1606. Asknown in the art, the truth table for T flip-flop 1606 specifies thatthe output Q change its state whenever the input is a “1” and that theoutput Q remain the same whenever the input is a “0”. The T flip-flop1606 also provides a complementary output Q′ that is the inverse of theoutput Q. Accordingly, if the differential encoder 1600 is used inconjunction with the preferred embodiment of FIG. 10, supra, it is notrequired that the differential encoder output signal x′(t) be split intotwo paths. Rather, the first output Q of the differential encoder 1600may be provided directly to the phase modulator 1005, and thecomplementary output Q′ may be provided directly to the electricalsignal splitter 1045, there being no need for the electrical inverter1044. The NRZ-RZ gate 1602, available from NEL Electronics of Tokyo,Japan, is designed to receive a sinusoid at frequency R at a firstinput, the NRZ data signal at bit rate R at a second input, and toproduce an electrical RZ-encoded version of the data at an output 1604.The T flip-flop 1606 is triggered at the falling edge of a “1” pulse tocause a toggle in the Q and Q′ outputs.

[0082] FIGS. 17-20 each illustrate a simplified block diagram of an RZsignal generator with multiple components integrated onto one or morecommon substrates in accordance with a preferred embodiment. Integrationon to as few substrates as possible provides advantages including loweroverall production costs and reduced optical losses associated withdiscrete device interfaces. The examples of FIGS. 17-20 provide for theinclusion of a frequency shift compensating phase modulator (PM) asdescribed supra with respect to FIG. 10, along with its associatedelectronics. In alternative preferred embodiments, this element may ofcourse be omitted if frequency shift in the output optical signal ispermissible, making the overall device even more compact.

[0083]FIG. 17 illustrates an RZ signal generator 1700 in accordance witha preferred embodiment comprising a lithium niobate substrate 1704 uponwhich is integrated a first PM 1706, a Mach-Zehnder interferometer (MZI)1708, and a second PM 1710. Electrical circuitry for driving the opticalcomponents is provided on a separate substrate 1712, as lithium niobateis an electrically insulating material. The first PM 1706 and MZI 1708perform functions similar to those of the phase modulator 405 andinterferometer 412 of FIG. 4, respectively, while the second PM 1708performs frequency shift compensation similar to the phase modulator1040 of FIG. 10. A CW laser 1702 that generates the initial carriersignal is provided on a separate substrate. A dotted line is shownbetween the electrical circuitry 1712 and the MZI 1708 to indicate thatthe MZI 1708 generally does not require any modulation/control signalsexcept for the embodiments of FIGS. 13-15, supra, in which the delay armof the MZI is precisely regulated.

[0084]FIG. 18 illustrates an RZ signal generator 1800 in accordance witha preferred embodiment comprising a semiconductor substrate 1804 uponwhich is integrated a first PM 1806, an MZI 1808, and a second PM 1810.Because the semiconductor substrates are amenable to fabrication ofelectrical circuits therein, electrical circuitry 1812 for driving theoptical components is provided on the same substrate as the opticalcomponents. A CW laser 1802 is provided on a separate substrate. Thesemiconductor substrate 1804 preferably has a zinc blend crystalstructure, and in a preferred embodiment comprises a III-V semiconductorsubstrate. In one preferred embodiment, the RZ signal generator 1800 maycomprise a GaAs/AlGaAs material system, while in another preferredembodiment it may comprise a InP/InGaAsP material system.

[0085]FIG. 19 illustrates an RZ signal generator 1900 in accordance witha preferred embodiment comprising a semiconductor substrate 1904 uponwhich is integrated a CW laser 1902, a first PM 1906, an MZI 1908, asecond PM 1910, and electrical circuitry 1912 for driving the opticalcomponents. RZ signal generator 1900 further comprises a variableoptical amplifier or attenuator (VOA) 1907 positioned to receive theoutput of PM 1906, and to provide an amplified or attenuated version ofthe optical signal to the MZI 1908. The VOA 1907 may provide lossrecovery and/or signal equalization functionalities. By integrating theVOA 1907 onto the same chip as the other components in accordance withthe preferred embodiments, a single-chip device may provide opticalgeneration, modulation, and equalization of an optical channel that maythen be combined with other optical channels into a WDM optical signal.The equalization function, of course, will require external monitoringof all WDM channels and external provision of an equalization controlsignal to the VOA 1907. According to a preferred embodiment, the VOA1907 may comprise a semiconductor optical amplifier (SOA) havingmultiple transverse cavities for exciting the gain medium thereof, asdescribed in U.S. patent application Ser. No. ______, entitled“SEMICONDUCTOR OPTICAL AMPLIFIER WITH TRANSVERSE LASER CAVITYINTERSECTING OPTICAL SIGNAL PATH AND METHOD OF FABRICATION THEREOF,”Attorney Docket No. 0980/65847, filed on the same day as thisapplication, which is incorporated by reference herein. Advantageously,such transversely excited SOA device may be operated in either anamplification mode or an attenuation mode, thereby increasing theversatility of the RZ signal generator 1900. Alternatively, the VOA 1907may comprise an evanescently excited SOA in which the gain medium of theSOA is evanescently excited by a nearby lasing field. In still anotherpreferred embodiment, the VOA 1907 may be provided in the form of atunable coupler, in which case only attenuation/equalizationfunctionality would be provided by the VOA 1907.

[0086] Although the VOA 1907 is shown between the PM 1906 and MZI 1908for better noise performance, it may alternatively be positioned beforethe PM 1906 or after the PM 1910. In the latter configuration, in apreferred embodiment in which the VOA 1907 comprises a transversely orevanescently excited SOA device, it may be driven to provide “chirp” tothe optical output signal, which may be useful in certain pulsecompression applications. When the transversely or evanescently excitedSOA device is placed after the MZI 1908, self-phase modulation to the RZpulses due to the carrier depletion induced by the optical signal willmake the RZ pulses chirped. For this, it is preferable that thetransversely or evanescently excited SOA device has a fast enoughcarrier recovery speed in order to suppress the inter-symbol crosstalk.

[0087]FIG. 20 illustrates an RZ signal generator 2000 in accordance witha preferred embodiment in which a first set of optical components isintegrated onto a semiconductor substrate and a second set of opticalcomponents is integrated onto a lithium niobate substrate. RZ signalgenerator 2000 comprises a semiconductor substrate 2004 upon which isintegrated a CW laser 2002, a first PM 2006, a VOA 2007, a second PM2010, and electrical driving circuitry 2012A. An MZI 2008 is provided ona separate lithium niobate substrate, and is configured to induce achange in direction of the optical signal between its input and outputso that the other components on semiconductor substrate 2004 may be morecompactly arranged. Electrical circuitry 2012B is separately provided todrive the MZI 2008, or alternatively this circuitry may be integratedinto the electrical driving circuitry 2012A on the semiconductorsubstrate 2004.

[0088]FIG. 21 illustrates an optical integrated circuit 2100 inaccordance with a preferred embodiment, which operates as an RZ signalgenerator when provided with an optical carrier from an optical source(not shown) and appropriate electronic control circuitry (not shown).The incorporation of optical components onto an integrated substratesuch as LiNb₃, GaAs, or InP can often result in excessively large devicelengths, e.g., on the order of 10-20 cm or more, when the opticalcomponents are placed in a linear arrangement as in FIGS. 17-19, supra.This is because the individual optical components such as phasemodulators, delay elements, etc., often require several centimeters oflength each to achieve their prescribed functionalities. On the otherhand, if the optical components are arranged vertically (i.e.,side-by-side), the optical path being S-shaped with the optical pathbeing turned 180 degrees between optical components over curvedwaveguides, there can be excessive bending losses if the radius ofcurvature of any curved waveguide is less than 1 cm. Because the widthsof the optical components themselves are very small, on the order ofabout 10-20 μm, both of the above scenarios can result in a substantialamount of wasted or unused substrate area, which increases overalldevice size and cost. It would be desirable to provided an integratedoptical circuit such as an RZ signal generator in which there is moreefficient use of substrate area.

[0089] According to a preferred embodiment, a folded waveguide structureis used to couple successive optical components of the RZ signalgenerator, thereby substantially reducing the amount of lateralseparation between optical components and reducing the overall devicesize. Optical integrated circuit 2100 comprises a substrate 2104 such asLiNb₃, GaAs, or InP, on which is formed a first phase modulator 2106, anMZI 2108, and a second phase modulator 2110 having functionalitiessimilar to like elements shown in FIG. 17. Antireflective coatings 2112and 2114 are provided at the input 2101 and output 2103, respectively. Afolded waveguide structure 2107 is used to couple the first phasemodulator 2106 to the MZI 2108, in which optical waveguides converge atshallow angles (e.g., 1-5 degrees) and terminate at areflectivity-enhanced surface 2116 such as a mirror. The foldedwaveguide structures 2107 are configured and dimensioned such that mostof the light exiting the first phase modulator 2106 is directed onwardtoward the MZI 2108. It has been found that very close spacing, on theorder of a few millimeters or less, can be achieved between the firstphase modulator 2106 and the MZI 2108 using the folded waveguidestructure 2107. FIG. 21 also shows another folded waveguide structure2109 including a reflectivity-enhanced surface 2118 for coupling the MZI2108 to the second phase modulator 2110. Suitable couplingconfigurations and angles for the folded waveguide structures 2107 and2109 can be found in U.S. Pat. No. 6,243,516, which is incorporated byreference herein. The reflectivity-enhanced surfaces 2116 and 2118should be flat and precisely oriented with respect to the convergingoptical waveguides. Advantageously, most conventional optical integratedcircuit fabrication techniques already provide for the formation of veryflat and straight substrate edges for enabling efficient coupling withexternal optical fibers. Thus, very flat and smooth reflective surfacesare “automatically” provided for forming precise reflectivity-enhancedsurfaces 2116 and 2118.

[0090]FIG. 22 illustrates an optical integrated circuit 2200 inaccordance with a preferred embodiment, which is shown as a genericoptical device comprising optical components 2206, 2208, and 2210.According to a preferred embodiment, when the functionality of aninterferometer is required in an optical integrated circuit, adual-function edge interferometer can be used to achieve the combinedfunctions of an interferometer and a folded waveguide structure, therebyfurther increasing efficiency of substrate use. Optical integratedcircuit 2200 comprises a substrate 2204, a folded waveguide structure2209 with mirror 2218 similar to elements 2109 and 2118 of FIG. 21,respectively, and antireflective surfaces 2212 and 2214 at the deviceinput 2201 and output 2203, respectively. A dual-function edgeinterferometer 2220 is formed in the optical path between the firstelement 2206 and the third optical element 2208. Dual-function edgeinterferometer 2220 is a Michelson interferometer, containing a singlecoupler 2222, a first arm 2224, a second arm 2226, and first and secondmirrors 2228 and 2230, respectively. As indicated in FIG. 22, an edge ofthe substrate 2204 is shaped such that the first arm 2224 exceeds thesecond arm 2226 in length by a distance ΔL. By way of example, if therequired fixed time delay difference τ is 10 ps, and if the speed oflight in the substrate is c/3, the distance ΔL would be 0.5 mm. The edgeof the substrate 2204 can be processed using known methods to achievethe spacing ΔL. An electrically controlled element 2234 may be placednear one of the interferometer arms to allow for precise fine-tuning ofthe time delay τ. Because the dual-function edge interferometer 2220 isa Michelson interferometer, light will be reflected back into the sourcewhen there is destructive interference at the output. Accordingly, anisolator 2232 is provided near the input to assure that light is notreflected back into the carrier source.

[0091] Accordingly, the dual-function edge interferometer 2220represents an integrated optical circuit for interferometricallyredirecting an optical signal from propagation in a first direction on afirst waveguide coming from the first optical component 2206, topropagation in a second direction on a second waveguide leading to thethird optical component 2208, the second direction being approximatelyopposite the first direction. The second waveguide is separated from thefirst waveguide by less than a minimum bending radius corresponding tothe material system of the substrate 2204 upon which the integratedoptical circuit is formed. The integrated optical circuit comprises anoptical coupler 2222 having an input coupled to the first waveguide, anoutput coupled to the second waveguide, a first intermediate port, and asecond intermediate port. The integrated optical circuit furthercomprises a first reflective surface 2228 formed on a first edge of thesubstrate, a first interferometer arm 2224 coupled between the firstintermediate port and the first reflective surface, a second reflectivesurface 2230 formed on a second edge of the substrate, and a secondinterferometer arm 2226 coupled between the second intermediate port andthe second reflective surface. The first and second substrate edges arepositioned with respect to the optical coupler such that an effectivepath length of said first interferometer arm differs from an effectivepath length of said first interferometer arm by a predetermined amountcorresponding to a desired interferometric time delay τ. The first andsecond substrate edges form a step-like indentation along a major edgeof the substrate, i.e. the right edge of the substrate 2204 in FIG. 22.

[0092] Whereas many alterations and modifications of the presentinvention will no doubt become apparent to a person of ordinary skill inthe art after having read the foregoing description, it is to beunderstood that the particular embodiments shown and described by way ofillustration are in no way intended to be considered limiting. Forexample, in another preferred embodiment, the phase modulators used maycomprise semiconductor optical amplifiers (SOA) configured using knownmethods to achieve phase modulation. As another example, while the phasemodulators supra are described as being driven by electrical drivingsignals, any of a variety of devices capable of phase modulation may beused, including devices that may be driven by electrical, mechanical,acoustical, optical, or other types of driving signals. As anotherexample, while the substrate edges containing the mirrors of thedual-function edge interferometer of FIG. 22 are shown as beingparallel, any of a variety of mirror orientations and/or waveguideconfigurations may be used to achieve the required interferometer armpath differences, including those described in U.S. Pat. No. 6,104,847,which is incorporated by reference herein. Therefore, reference to thedetails of the preferred embodiments are not intended to limit theirscope, which is limited only by the scope of the claims set forth below.

What is claimed is:
 1. An apparatus for generating return-to-zero (RZ)optical pulses corresponding to an information signal, comprising: aphase modulator for causing a phase change in an optical carrier signalresponsive to a transition in a driving signal derived from theinformation signal; and an interferometer coupled to receive an outputof said phase modulator, said interferometer causing a fixed time delaybetween first and second signals derived from said output of said phasemodulator, said fixed time delay being selected such that said first andsecond signals destructively interfere when no phase change is occurringin said output of said phase modulator and such that said first andsecond signals do not destructively interfere when said phase changedoes occur, an output of said interferometer comprising RZ opticalpulses corresponding to said transitions in said driving signal.
 2. Theapparatus of claim 1, wherein said fixed time delay is set to an integermultiple of a period of the optical carrier signal, and wherein saidinterferometer output corresponds to a subtractive combination of saidfirst and second signals.
 3. The apparatus of claim 1, wherein saidfixed time delay is set to an integer multiple of a period of theoptical carrier signal plus one-half of said period, and wherein saidinterferometer output corresponds to an additive combination of saidfirst and second signals.
 4. The apparatus of claim 1, wherein saiddriving signal and said information signal are electrical signals. 5.The apparatus of claim 1, further comprising a differential encoder forreceiving said information signal and generating said driving signaltherefrom, said output of said interferometer having a binary patternequal to a binary pattern of said information signal.
 6. The apparatusof claim 5, said RZ optical pulses at said output of said interferometerhaving an induced frequency shift, said apparatus further comprising aphase modulating element at the output of said interferometer forimposing a compensating frequency shift on said RZ optical pulses, saidcompensating frequency shift being opposite in sign and substantiallyequal in magnitude to the induced frequency shift.
 7. The apparatus ofclaim 6, wherein said phase modulator, said interferometer, and saidphase modulating element are integrated onto a common substrate having amaterial system selected from the group consisting of: lithium niobate,semiconductor, InP, and GaAs.
 8. The apparatus of claim 6, said drivingsignal and said information signal being electrical signals, said phasemodulating element being driven by a first electrical signal derivedfrom said driving signal.
 9. The apparatus of claim 8, furthercomprising: an electrical splitter for splitting said driving signalinto second and third electrical signals; an electrical delay elementfor delaying said third electrical signal with respect to said secondelectrical signal by an amount substantially equal to said fixed timedelay of said interferometer; and an electrical combining element forcombining said second and third electrical signals to form said firstelectrical signal.
 10. The apparatus of claim 1, wherein said drivingsignal is proportional to said information signal, said output of saidinterferometer having a binary pattern equal to a differentially encodedversion of a binary pattern of said information signal.
 11. Theapparatus of claim 1, further comprising: an optical source forproviding said optical carrier signal at a carrier frequency; and afeedback control circuit for precisely regulating either or both of (i)said fixed time delay of said interferometer, and (ii) said carrierfrequency of said optical source.
 12. The apparatus of claim 11, saidfeedback control circuit comprising: a detector coupled to an auxiliaryoutput of said interferometer, said detector measuring an averageoptical power at said auxiliary output; and a control circuit coupled tosaid detector and to a fixed time delay element of said interferometer,said control circuit manipulating either or both of said fixed timedelay and said carrier frequency such that said average optical power ismaintained at an extremum.
 13. The apparatus of claim 11, said opticalcarrier signal being polarized at a first polarization angle, saidfeedback control circuit comprising: a coupler for tapping a pilot beamfrom said optical carrier signal prior to phase modulation of saidoptical carrier signal; a 90-degree rotating element coupled to saidcoupler for causing said pilot beam to be at a second polarization anglethat is 90-degrees from said first polarization angle; a firstpolarization beamsplitter positioned between said phase modulator andsaid interferometer for combining said pilot beam with said phasemodulator output, said pilot beam not interacting with said phasemodulator output in said interferometer due to said 90-degreepolarization difference, said output of said interferometer comprisingsaid pilot beam at said second polarization angle and said RZ opticalpulses at said first polarization angle, said pilot beam having anaverage power level that is at an extremum when said fixed time delay ofsaid interferometer is at an optimal value; a second polarizationbeamsplitter for receiving said interferometer output and extractingsaid pilot beam therefrom; a detector coupled to said polarizationbeamsplitter for receiving said pilot beam and measuring said averagepower level thereof; and a control circuit coupled to said detector andto a fixed time delay element of said interferometer, said controlcircuit manipulating either or both of said fixed time delay and saidcarrier frequency such that said average optical power of said pilotbeam is maintained at said extremum.
 14. The apparatus of claim 1,wherein said fixed time delay is set approximately equal to an averageduration of said transitions of said driving signal.
 15. The apparatusof claim 1, wherein said phase modulator and said interferometer areintegrated onto a common substrate having a material system selectedfrom the group consisting of: lithium niobate, semiconductor, InP, andGaAs.
 16. The apparatus of claim 15, further comprising a foldedwaveguide structure formed at an edge of said common substrate forcoupling said interferometer to said phase modulator.
 17. The apparatusof claim 15, said interferometer being a Michelson interferometer, saidinterferometer comprising: an optical coupler; a first arm between saidoptical coupler and a first mirror formed along a first edge of saidsubstrate; and a second arm between said optical coupler and a secondmirror formed along a second edge of said substrate; wherein said firstand second edges are positioned with respect to said optical coupler soas to achieve a predetermined path difference between said first arm andsaid second arm corresponding to said fixed time delay.
 18. Theapparatus of claim 15, said common substrate being a semiconductorsubstrate, further comprising a variable-gain optical element integratedonto said common substrate, said variable-gain optical elementcomprising a semiconductor optical amplifier having a gain medium thatis excited by lasing fields oriented in a direction different than adirection of signal propagation therethrough.
 19. A method forgenerating return-to-zero (RZ) optical pulses corresponding to aninformation signal, comprising: generating a driving signal from saidinformation signal, said driving signal having two or more levels, saiddriving signal having level transition intervals of finite duration;generating a phase-modulated optical signal from an optical carriersignal by causing phase changes therein during said level transitionintervals of said driving signal; generating first and second opticalsignals from said phase-modulated optical signal, said second signalbeing a substantially identical but delayed version of said firstsignal, said second signal being delayed with respect to said firstsignal by an unmodulated, predetermined, fixed time delay τ; andcombining said first and second optical signals to produce a resultantoptical signal; wherein said fixed time delay τ is selected such thatsaid first and second optical signals destructively combine when nophase change is occurring in said phase-modulated optical signal, andsuch that said first and second optical signals do not destructivelyinterfere when said phase change does occur, whereby said resultantoptical signal comprises RZ optical pulses during said level transitionintervals of said driving signal.
 20. The method of claim 19, whereinsaid fixed time delay is an integer multiple of a period of the opticalcarrier signal, and wherein said step of combining comprises the step offorming a subtractive combination of said first and second opticalsignals.
 21. The method of claim 19, wherein said fixed time delay is aninteger multiple of a period of the optical carrier signal plus one-halfof said period, and wherein said step of combining comprises the step offorming an additive combination of said first and second opticalsignals.
 22. The method of claim 19, wherein said driving signal isgenerated by differentially encoding said information signal, said RZoptical pulses having a binary pattern equal to a binary pattern of saidinformation signal.
 23. The method of claim 22, said RZ optical pulseshaving an induced frequency shift, said method further comprisingimposing a compensating frequency shift on said RZ optical pulses, saidcompensating frequency shift being opposite in sign and substantiallyequal in magnitude to the induced frequency shift.
 24. The method ofclaim 23, wherein said driving signal and said information signal areelectrical signals, and wherein imposing a compensating frequency shiftcomprises: splitting the driving signal into first and second electricalsignals; delaying the second electrical signal with respect to the firstelectrical signal by an amount substantially equal to the fixed timedelay τ; of said interferometer; combining the first and secondelectrical signals to form a third electrical signal; andphase-modulating said RZ optical pulses with a phase modulator driven bythe third electrical signal.
 25. The method of claim 19, wherein thedriving signal is proportional to the information signal, the RZ opticalpulses having a binary pattern equal to a differentially encoded versionof a binary pattern of the information signal.
 26. The method of claim19, further comprising precisely regulating either or both of (i) saidfixed time delay τ, and (ii) a carrier frequency of said optical carriersignal to an optimal value.
 27. The method of claim 26, furthercomprising: measuring an average optical power of a complementaryresultant signal associated with said step of combining; andmanipulating either or both of said fixed time delay τ and said carrierfrequency such that said average optical power of the complementaryresultant signal is maintained at an extremum.
 28. The method of claim26, the optical carrier signal being polarized at a first polarizationangle, further comprising: tapping a pilot beam from the optical carriersignal prior to said step of generating a phase-modulated opticalsignal; rotating the pilot beam to a second polarization angle that is90-degrees from the first polarization angle; combining the pilot beamwith said phase-modulated optical signal prior to said step ofgenerating first and second optical signals, the resultant opticalsignal comprising the pilot beam at the second polarization angle andthe RZ optical pulses at the first polarization angle; extracting thepilot beam from the resultant optical signal; measuring an average powerlevel of the pilot beam; and manipulating either or both of said fixedtime delay τ and said carrier frequency such that the average opticalpower of the pilot beam is maintained at an extremum.
 29. The method ofclaim 19, wherein the fixed time delay τ is set approximately equal toan average level transition interval of the driving signal.
 30. Anapparatus, comprising: a first optical device having a first input forreceiving an optical carrier signal, a second input for receiving afirst voltage, and an output, said first optical device being capable ofinducing a variable phase change in the optical carrier signalproportional to the first voltage for generating a phase-modulatedoptical signal at said output thereof; a second optical device having aninput for receiving the phase-modulated signal and an output, saidsecond optical device being capable of splitting the phase modulatedoptical signal into first and second optical signals, the second opticaldevice being capable of inducing a time delay between the first andsecond optical signals and providing a combination thereof at saidoutput of said second optical device; wherein the time delay induced bythe second optical device is a fixed, predetermined, unmodulated timedelay that is an integer multiple of one-half the period of the opticalcarrier signal; wherein, when said first voltage is at a constant value,said output of said second optical device has a null amplitude; andwherein, when said first voltage is experiencing a change asapproximated over an interval equal to said time delay of said secondoptical device, said output of said second optical device has an activeamplitude.
 31. The apparatus of claim 30, further comprising: aninformation signal input for receiving a binary information signal; anda differential encoder having an input coupled to said informationsignal input and an output, said output being a binary signal having afinite transition time between levels, said output being coupled to saidsecond input of said first optical device; whereby said output of saidsecond optical device comprises an RZ-formatted optical signal having abinary pattern equal to the binary pattern of said information signal,each active bit of the RZ-formatted optical signal having a total pulsewidth approximately equal to a sum of said time delay of said secondoptical device and said transition time of said output of saiddifferential encoder.
 32. The apparatus of claim 30, further comprising:an information signal input for receiving a binary information signal,said information signal having finite transition times between levels,said information signal input being coupled to said second input of saidfirst optical device, wherein said first voltage is proportional to saidinformation signal; whereby said output of said second optical devicecomprises an RZ-formatted optical signal having a binary pattern equalto the binary pattern of said information signal, each active bit of theRZ-formatted optical signal having a total pulse width approximatelyequal to a sum of said time delay of said second optical device and saidtransition time of said output of said differential encoder.
 33. Theapparatus of claim 30, wherein said first optical device comprises aphase modulator.
 34. The apparatus of claim 33, wherein said secondoptical device comprises an interferometer.
 35. An apparatus forgenerating return-to-zero (RZ) optical pulses corresponding to aninformation signal, comprising: a differential encoder having an inputfor receiving the information signal and an output; a first variablephase changing element having a first input for receiving an opticalcarrier signal, a second input coupled to the output of saiddifferential encoder, and an output, said first variable phase changingelement inducing a phase change that monotonically corresponds to avoltage at said second input; an optical splitting element having aninput coupled to the output of said first variable phase changingelement, a first output, and a second output; an optical combiningelement having a first input, a second input, and an output; a firstoptical path between said first output of said optical splitting elementand said first input of said optical combining element, said firstoptical path inducing a first fixed time delay therebetween; a secondoptical path between said second output of said optical splittingelement and said second input of said optical combining element, saidsecond optical path inducing a second fixed time delay therebetween,wherein said first and second time delays differ by integer multiple ofone-half the period of the optical carrier signal, said output of saidoptical combining element comprising the RZ optical pulses correspondingto the information signal.
 36. The apparatus of claim 35, said RZoptical pulses at said output of said optical combining element havinginduced frequency shift, said apparatus further comprising: a secondvariable phase changing element having a first input coupled to saidoutput of said optical combining element, a second input, and an output,said second variable phase changing element inducing a phase change thatmonotonically corresponds to a voltage at said second input; anelectrical compensating circuit having an input coupled to the output ofsaid differential encoder and an output coupled to said second input ofsaid second variable phase changing element, said electricalcompensating circuit comprising: an inverter having an input coupled tosaid differential encoder output, and an output; an electrical splittingelement having an input coupled to the output of said inverter, a firstoutput, and a second output; an electrical combining element having afirst input, a second input, and an output, said output being coupled tosaid second input of said second variable phase changing element; afirst electrical path between said first output of said electricalsplitting element and said first input of said electrical combiningelement, said first electrical path inducing a third fixed time delaytherebetween; a second electrical path between said second output ofsaid electrical splitting element and said second input of saidelectrical combining element, said second electrical path inducing afourth fixed time delay therebetween, wherein said third and fourth timedelays differ by an amount approximately equal to said differencebetween said first and second time delays; whereby said output of saidsecond variable time delay comprises RZ optical pulses substantiallyequal to said RZ optical pulses at said output of said optical combiningelement but with substantially reduced frequency shift.
 37. Theapparatus of claim 36, wherein said differential encoder, said firstvariable phase changing element, said optical splitting element, saidoptical combining element, said first and second optical paths, saidsecond variable phase changing element, and said electrical compensatingcircuit are integrated onto a common semiconductor substrate.
 38. Theapparatus of claim 37, further comprising a variable-gain opticalelement integrated onto said semiconductor substrate.
 39. The apparatusof claim 38, said variable-gain optical element comprising asemiconductor optical amplifier having a gain medium that is excited bylasing fields oriented in a direction different than a direction ofsignal propagation therethrough.
 40. The apparatus of claim 35, whereinsaid first variable phase changing element, said optical splittingelement, said optical combining element, and said first and secondoptical paths are integrated onto a common substrate having a materialsystem selected from the group consisting of: lithium niobate,semiconductor, InP, and GaAs.
 41. The apparatus of claim 40, whereinsaid first variable phase changing element is coupled to said opticalsplitting element by a folded waveguide structure formed at an edge ofsaid common substrate.
 42. An integrated optical circuit forinterferometrically redirecting an optical signal from propagation in afirst direction on a first waveguide to propagation in a seconddirection on a second waveguide, the second direction beingapproximately opposite the first direction, the second waveguide beingseparated from the first waveguide by less than a minimum bending radiuscorresponding to a substrate upon which the integrated optical circuitis formed, the integrated optical circuit comprising: an optical couplerhaving an input coupled to the first waveguide, an output coupled to thesecond waveguide, a first intermediate port, and a second intermediateport; a first reflective surface formed on a first edge of thesubstrate; a first interferometer arm coupled between said firstintermediate port and said first reflective surface; a second reflectivesurface formed on a second edge of the substrate; and a secondinterferometer arm coupled between said second intermediate port andsaid second reflective surface; wherein said first and second substrateedges are positioned with respect to said optical coupler such that aneffective path length of said first interferometer arm differs from aneffective path length of said first interferometer arm by apredetermined amount corresponding to a desired interferometric timedelay.
 43. The integrated optical circuit of claim 42, wherein saidfirst and second substrate edges are substantially parallel to eachother.
 44. The integrated optical circuit of claim 43, wherein saidfirst and second interferometer arms are substantially parallel to eachother and to said first and second waveguides.
 45. The integratedoptical circuit of claim 44, wherein said first and secondinterferometer arms are separated by a distance not exceeding saidminimum bending radius along their entire lengths.
 46. The integratedoptical circuit of claim 45, wherein said first and second substrateedges form a step-like indentation along a major edge of the substrate.47. The integrated optical circuit of claim 45, wherein said substrateis lithium niobate, and wherein said minimum bending radius isapproximately 1 cm.
 48. The integrated optical circuit of claim 45,further comprising a resistive heating element positioned near saidfirst interferometer arm for allowing fine-tuning of its effective pathlength.
 49. The integrated optical circuit of claim 48, wherein saidoptical coupler, said first and second interferometer arms, and saidfirst and second reflective surfaces form a Michelson interferometer.